The present invention relates to a band switching filter using a surface acoustic wave resonator and an antenna duplexer using the same.
An antenna duplexer of the prior art is generally formed by a distributed constant circuit type resonator such as a coaxial line resonator and number of stages and a construction of filters of a resonator have been determined based on pass bandwidth and attenuation quantity.
The problem in the above antenna duplexer is that it difficult to make the size small because a filter characteristic largely depends on a form of the resonator. For example, a similar problem has existed, even if it has a construction of a band switching filter using a distributed constant circuit type resonator and switching diodes, like Japanese Unexamined Patent 10-150304.
The present invention aims to miniaturize such a band switching filter and an antenna duplexer using it.
To address this problem, a band switching filter of the present invention is a filter selectively passing or attenuating either one of different two bandwidths and is characterized by that at least one of two resonant frequencies of the surface acoustic wave resonators is moved to an arbitrary resonant frequency, using resonator circuits each composed of at least a surface acoustic wave resonator, a switching element and an impedance element and switching a connection state between the surface acoustic wave resonator and the impedance element through on/off of the switching element.
In accordance with the above, not only the resonator can be miniaturized but also number of stages of filters can be reduced by switching the bandwidth. As a result, the band switching filter can be miniaturized.
Also an antenna duplexer can be miniaturized by using at least one of the above mentioned band switching filter.